The point is this: Your body won't get used to running long distances, unless it has run those distances on a regular basis. But, the body needs rest between those runs, which is why we suggest no more than two long runs per week and moderate distance on the other days. At the beginning of your training program, those long runs could be 6 miles each. Then, as the weeks go by, gradually increase them. Perhaps week two would see the long runs as 6 & 8 miles, week three 7 & 9, week four: 7 & 10, etc. Gradually increase these until your two runs are closer to 12 and 18 miles. At this point you have built an excellent base. from marathonguide.com